For the purposes of the present invention, nitrile rubbers, also referred to as “NBRs” for short, are rubbers which are copolymers or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and optionally one or more further copolymerizable monomers.
The storage stability of such nitrile rubbers is frequently problematical. For the present purposes, storage-stable means that the Mooney viscosity as important specification criterion for nitrile rubbers changes very little during prolonged storage times and in particular also at high temperatures as can appear in summer.
Nitrite rubbers and processes for producing such nitrile rubbers are known, see, for example, W. Hofmann, Rubber Chem. Technol. 36 (1963) 1 and Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft, Weinheim, 1993, pp. 255-261. This publication gives no indication as to whether and if appropriate how the storage stability of nitrile rubbers can be improved.
JP 75,105,746 describes heat-resistant nitrile rubbers which are obtained by carrying out the coagulation of the latex by means of a mixture of tin dichloride and calcium chloride. Here, 50 parts by weight of tin dichloride are used per 100 parts by weight of calcium chloride. The use of tin salts is nowadays problematical for ecological reasons, especially since these tin salts are found in the nitrile rubber even after comprehensive subsequent washing of the nitrile rubber. The removal of the tin salts from the washing water is also associated with a high and therefore likewise undesirable outlay for purification.
It is known from JP 76/26,790 that a nitrile rubber latex can be coagulated using strontium chloride, which results in a coagulated nitrile rubber having a strontium content of 1.2%. It is stated that a moulding produced on the basis of such a nitrile rubber has significantly better properties than a corresponding moulding based on a nitrile rubber obtained from the latex by coagulation using calcium chloride.
According to Angew. Makromol. Chem. 1986, 145-146, 161-179, an extremely effective measure for improving the storage stability of nitrile rubber is selective hydrogenation of the double bonds originating from the butadiene while at the same time retaining the triple bonds of the nitrile groups. The property changes achieved by the hydrogenation are desirable for many applications, but not for all. In addition, the hydrogenation is complicated and requires a series of additional process steps. In addition, the glass transition temperatures are usually made worse compared to unhydrogenated starting material by the hydrogenation. For this reason, hydrogenation is not a suitable solution to the problem for all applications.
NBR is produced by emulsion polymerization, which firstly gives an NBR latex. The NBR solid is isolated from this latex by coagulation. Salts and acids are used for coagulation. In the coagulation of latices by means of metal salts, it is known that significantly larger amounts of electrolyte are required in the case of monovalent metal ions, e.g. in the form of sodium chloride, than in the case of polyvalent metal ions, e.g. in the form of calcium chloride, magnesium chloride or aluminium sulphate (Kolloid-Z. 154, 154 (1957)). It is also known from Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 484 that the use of polyvalent metal ions leads to “at least some inclusion of the emulsifier in the product”. According to Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, p. 479, “not only do the electrolytes used have to be very carefully washed out again, but the finished product should also he free of the catalysts and emulsifiers of the process batch. Even small amounts of residual electrolytes give turbid and cloudy pressed and injection-moulded parts, impair the electrical properties and increase the water absorption capacity of the finished product” (citation). However, Houben-Weyl gives no indication as to whether and in what way the work-up of the latex influences its storage stability.
DD 154 702 discloses a process for the free-radical copolymerization of butadiene and acrylonitrile in emulsion, which is controlled by means of a specific, advantageously computer-aided metering program for the monomers and the molecular weight regulators, e.g. tert-dodecyl mercaptan, and in which the latices obtained are worked up by coagulation in an acid medium to give the solid rubber. A significant advantage of the process is said to be that the resin soaps and/or fatty acid soaps used as emulsifiers remain in the rubber as a result of the use of acids in the coagulation. i.e. they are not washed out as in the case of other processes. In addition to the advantage of good properties of the NBR, the improvement in the economics of the process and the avoidance of wastewater pollution by washed-out emulsifier are specifically advertised here. It is stated that the butadiene-acrylonitrile copolymers containing 10-30% by weight of acrylonitrile obtained have good elasticity and low-temperature properties combined with an increased swelling resistance and advantageous processability. Measures by means of which the storage stability of the nitrile rubber can be influenced are not revealed by the teachings of this patent.
JP 27902/73 (Appl. 69 32,322) discloses that the use of amines in the coagulation of latices by means of magnesium salts, for example by means of a combination of diethylenetriamine and magnesium chloride, enables the initial vulcanization rate to be reduced and thus the scorch resistance of nitrile rubbers to be improved. However, no information as to how storage-stable nitrile rubbers can be obtained is given.
DE-A 23 32 096 discloses that rubbers can be precipitated from their aqueous dispersions by means of methylcellulose and a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt. Preference is given to using sodium chloride as water-soluble salt. It is stated that an advantage of this process is that it gives a coagulum which is virtually completely free of extraneous constituents such as emulsifiers, catalysts residues and the like since these extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues are completely washed out by means of further water. Information about the storage stability of rubbers produced in this way is not given. In DE-A 24 25 441, the electrolyte coagulation of rubber latices is carried out using 0.1-10% by weight (based on the rubber) of water-soluble C2-C4 alkylcelluloses or hydroxyalkylcelluloses in combination with from 0.02 to 10% by weight (based on the rubber) of a water-soluble alkali metal, alkaline earth metal, aluminium or zinc salt as auxiliary instead of methylcellulose. Here too, preference is given to using sodium chloride as water-soluble salt. The coagulum is separated off mechanically, optionally washed with water and the remaining water is removed. Here too, it is stated that the extraneous materials are, as in DE-A 23 32 096, essentially completely removed together with the water when the coagulum is separated off and any remaining residues are washed out completely in the washing with further water.
In DE-A 27 51 786, it is established that the precipitation and isolation of rubbers from their aqueous dispersions can be carried out by means of a smaller amount of (hydroxy)alkylcellulose when from 0.02 to 0.25% by weight of a water-soluble calcium salt is used. A further advantage is said to be that this process gives an extremely pure coagulum which is essentially completely free of extraneous constituents such as emulsifiers, catalysts residues and the like. These extraneous materials are removed together with the water when the coagulum is separated off and any remaining residues can be washed out by means of water. It is also stated that the properties of the isolated rubbers are not adversely affected by a calcium salt being used for coagulation. Rather, it is said that a rubber whose vulcanization properties are not impaired and are fully satisfactory is obtained. This is presented as surprising since it is said that impairment of the rubber properties is frequently observed when polymers are precipitated from dispersions by means of polyvalent metal ions such as calcium or aluminium ions. Houben-Weyl (1961), Methoden der Org. Chemie, Makromolekulare Stoffe 1, pp. 484/485, is offered as evidence for the last statement. In contrast, the rubbers of DE-A 27 51 786 display no slowing or worsening of, for example, the initial vulcanization and/or full vulcanization.
None of the documents DE-A 23 32 096, DE-A 24 25 441 and DE-A 27 51 786 disclose which measures have to be taken in order to achieve high storage stability of nitrile rubbers.
As in the case of the above-described patents, the object of DE-A 30 43 688, is also to achieve a large reduction in the amounts of electrolyte required for coagulation of the latex. According to the teachings of DE-A 30 43 688, this is achieved by using either plant-based protein-like materials or polysaccharides such as starch and if appropriate water-soluble polyamine compounds as auxiliaries in addition to the inorganic coagulate in the electrolyte coagulation of latices. As inorganic coagulates, preference is given to alkali metal or alkaline earth metal salts. The specific additives make it possible to achieve a reduction in the amounts of salts used for quantitative coagulation of the latex. DE-A 3 043 688 gives no information as to how an improvement in storage stability can be achieved as a result of the production and/or work-up of the nitrile rubber.
In U.S. Pat. No. 4,920,176, it is stated and evidenced by experimental data that very high sodium, potassium and calcium contents and also emulsifiers remain in the nitrile rubber in coagulation of a nitrile rubber latex by means of inorganic salts such as sodium chloride or calcium chloride. However, this is undesirable and, according to the teachings of U.S. Pat. No. 4,920,176, water-soluble cationic polymers are used instead of inorganic salts in the coagulation of nitrile rubber latices for the purpose of obtaining very pure nitrile rubber. The polymers used here are, for example, ones based on epichlorohydrin and dimethylamine. These auxiliaries are used with the aim of significantly reducing the amounts of salts remaining in the product. The vulcanizates obtained therefrom display lower swelling on storage in water and an increased electrical resistance. In the patent text, the property improvements mentioned are attributed purely qualitatively to the minimal cation contents remaining in the product. A more detailed explanation of the phenomena observed is not given. U.S. Pat. No. 4,920,176 also gives no information as to whether and how the storage stability can be controlled by means of the production and work-up of the nitrile rubber.
The objective of EP-A-1 369 436 is to provide nitrile rubbers having a high purity. The process of EP-A-1 369 436 starts out from typical nitrile rubbers. Nothing is said about the polymerization process except that an emulsion polymerization is carried out in the presence of salts of fatty acids and/or resin acids as emulsifiers. This is followed by coagulation of the latex by means of acids, optionally with addition of precipitants. As acids, it is possible to use all mineral and organic acids which allow the desired pH values to be set. In addition, it is possible to use additional precipitates, with mention being made of alkali metal salts of inorganic acids, e.g. sodium chloride and sodium sulphate, for this purpose. The fatty acids and resin acids formed as a result of the action of acid are subsequently washed out by means of aqueous alkali metal hydroxide solutions and the polymer is finally subjected to shear until a residual moisture content of less than 20% is obtained. As a result of this shearing action, the water or the residual moisture including the ion contents and other foreign substances present therein are removed. The Ca contents of the products disclosed in Examples 1 and 2 are only 4 and 2 ppm, respectively. EP-A-1 369 436 gives no information on the production of nitrile rubbers which display increased storage stability.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 in each case describe nitrile rubbers based on an unsaturated nitrile and a conjugated diene. All the nitrile rubbers contain 10-60% by weight of unsaturated nitrile and have a Mooney viscosity in the range 15-150 or, according to EP-A-0 692 496, in the range 15-65 and all have at least 0.03 mol of C12-C16-alkylthio group per 100 mol of monomer units, with this alkylthio group having at least three tertiary carbon atoms and a sulphur atom which is bound directly to at least one of the tertiary carbon atoms. The nitrile rubbers are in each case produced in the presence of a C12-C16-alkyl thiol having a corresponding structure as molecular weight regulator which functions as “chain transfer agent” and is thus incorporated as end group into the polymer chains.
In the case of the nitrile rubbers of EP-A-0 779 300, it is stated that they have a width “ΔAN” (AN=unsaturated nitrile) of the composition distribution of the unsaturated nitrile in the copolymer in the range from 3 to 20. The process for producing them differs from that of EP-A-0 692 496 in that only 30-80% by weight of the total amount of monomers is used at the beginning of the polymerization and the remaining amount of monomers is fed in only at a conversion of the polymerizsation of 20-70% by weight.
In the case of the nitrile rubbers of EP-A-0 779 301, it is stated that they contain 3-20% by weight of a fraction having a low molecular weight and a number average molecular weight Mn of less than 35 000. The process for producing them differs from that of EP-A-0 692 496 in that only 10-95% by weight of the alkyl thiol are mixed into the monomer mixture before the polymerization and the remaining amount of the alkyl thiol is fed in only after a polymerization conversion of 20-70% by weight has been reached.
With regard to the coagulation of the latex, all three patent applications EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 disclose that any coagulants can be used. As inorganic coagulant, calcium chloride and aluminium chloride are mentioned and used. The focus is on nitrile rubbers which are essentially halogen-free and obtained by carrying out the coagulation of the latex in the presence of a non-ionic surface-active auxiliary and using halogen-free metal salts such as aluminium sulphate, magnesium sulphate and sodium sulphate. Coagulation using aluminium sulphate or magnesium sulphate is said to be preferred The resulting, essentially halogen-free nitrile rubber has a halogen content of not more than 3 ppm.
In Comparative Example 6 of EP-A-779 300 and Comparative Example 7 of EP-A-0 779 301, the coagulation of the latex is carried out using a mixture of NaCl and CaCl2, with the CaCl2 being used in large amounts and the weight ratio of NaCl to CaCl2 being 1:0.75. In respect of the scorching time and the stress at 100% elongation, no significant differences from the other examples shown in the respective Table 12 or 13 are found.
According to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is essential to use alkyl thiols in the form of the compounds 2,2,4,6,6-pentamethylheptane-4-thiol and 2,2,4,6,6,8,8-heptamethylnonane-4-thiol as molecular weight regulators for the production of the nitrile rubbers. It is clearly pointed out here that the use of the conventional known tert-dodecyl mercaptan as regulator gives nitrile rubbers having poorer properties.

In the case of the nitrile rubbers produced in EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is stated that they have an advantageous property profile, good processability of the rubber mixtures and make low fouling of the mould possible during processing. The vulcanizates obtained are said to have a good combination of low-temperature resistance and oil resistance and possess good mechanical properties. It is also stated that high polymerization conversions of greater than 75%, preferably greater than 80%, in the production of the nitrile rubbers enable a high productivity to be achieved and the vulcanization rate in vulcanization using sulphur or peroxides is high, in particular in the case of NBR grades for injection moulding. It is also indicated that the nitrile rubbers have a short initial vulcanization time and a high crosslinking density. Nothing is said about the property of storage stability in the patent applications mentioned.
In summary, it can be said that no process which makes it possible to synthesize nitrile rubbers which foreseeably have a good storage stability has been described up to now.